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    arthropod

    predatory_mite.jpg
    Pergamasus sp. A predatory mite of the type that preys on nematodes, springtails, other mites, and the larvae of insects. This mite is 1/25 of an inch (1mm) long. Credit: Gerhard Eisenbeis and Wilfried Wichard. 1987. Atlas on the Biology of Soil Arthropods. Springer-Verlag, New York. p. 83
    Any segmented invertebrate of the phylum Arthropoda, including the insects, arachnids (spiders, scorpions, etc), crustaceans, and myriapods. More than a million species are known, although according to some estimates there may be as many as 6 million species on Earth. More than 80 percent of all described species are arthropods, most of them insects.

    Arthropods are protostomes related to annelids. Among extinct forms are the trilobites.


    Characteristics

    All arthropods have a hard outer skin of chitin that is attached to the muscular system on the inside, and is periodically shed during growth. The body is divided into segments, modified among different groups, with each segment originally carrying a pair of walking or swimming jointed legs. In some animals some of the legs have evolved into jaws, sucking organs, or weapons. Arthropods have well-developed digestive, circulatory, and nervous systems. Land forms use tracheae for respiration.


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